This invention relates generally to asymmetric digital subscriber loop (ADSL) transceivers and more particularly to discrete multi-tone (DMT) asymmetric digital subscriber loop (ADSL) transceivers.
As is known in the art, ADSL modems achieve full-duplex operation over a single pair of subscriber loop (i.e., twisted pair telephone line) through the use of either frequency-division-multiplexing (FDM) or echo cancellation (EC). ADSL modems use DMT, a multi-carrier modulation technique, to achieve high bandwidth efficiency over a bandwidth of about 1.024 MHz (more precisely, 1.104 MHz). An ADSL transceiver system generally includes a modem at a central station, or office, adapted to transmit information in a down-stream signal to a modem at a remote terminal and to receive information in an up-stream signal transmitted by the modem at the remote terminal. The up-stream and down-stream signals pass through a common transmission medium, typically the twisted-pair telephone line. The up-stream signal comprises data carried by a lower portion of a band of frequencies extending over M, here 255, subchannels; i.e., the lower 31 subchannels. The down-stream signal comprises data carried by an upper portion of the 255 subchannels; i.e., the upper subchannels from subchannel 1 through 255). (The generation of these M subchannels may be with an N point Fast Fourier Transform (F.F.T.), where the N point F.F.T. provides M=(N/2)-1 subchannels; i.e., here N=512). Thus, this frequency band asymmetry is intended to accommodate a large down-stream signal data rate to support data-hungry applications such as video-on-demand and Internet access, and a low up-stream signal data rate for interactive control and basic-rate IDSN. This is implemented by employing an eight times wider bandwidth for the down-stream signal than for the up-stream signal. In terms of the multi-carrier modulation, the down-stream signal bandwidth consists of carrier subchannels 1 through 255 whereas the up-stream signal bandwidth consists of only carrier subchannels 1 through 31; in actual practice the first 6 to 8 carrier subchannels are used as a guard-band for plain ordinary telephone service (POTS). An EC based system makes use of the carrier allocation just mentioned, however, FDM systems avoid the overlap of up-stream and down-stream signal bandwidths by using a carrier assignment such as subchannels 35 through 255 for the down-stream subchannels and 8 through 31 for the up-stream subchannels, for example.
One such DMT FDM ADSL system 10, shown in FIG. 1, is adapted to exchange information between a modem 12 at a first station, here a central office (CO), and a modem 14 at a second station, here a remote terminal (RT), through a common communication medium 16, here a twisted-pair telephone line. The system 10 includes: a transmitter section 18, at the central office modem 12, for distributing a first stream of data on line 13 among a plurality of, M, (here 255) carrier frequencies shown in FIG. 2. More particularly, the transmitter section 18 of the central office modem 12 includes a modulator 20, here for receiving frames of the data on line 13 and for distributing such data over the upper portion of the plurality of, M, carrier frequencies. Here, the modulator 20 includes a Quadrature Amplitude Modulator (Q.A.M.) encoder 24 and an Inverse Fast Fourier Transformer (I.F.F.T.) 26 arranged in as conventional manner as shown. Here, the I.F.F.T. 26 is a 512 point I.F.F.T. Thus, the incoming data on line 13 is selectively encoded by the Q.A.M. encoder 24 at a frame rate, f.sub.r, here about 4 KHz (more precisely 4.0588 KHz) and the I.F.F.T. 26 produces for each frame a sequence of digital samples on line 22 at a rate f.sub.s =2(M+1)f.sub.r. More particularly, the sequence of digital samples on line 13 is encoded by Q.A.M. encoder 24 onto the 256 input lines 28.sub.0 -28.sub.511 of the I.F.F.T. 26 as a sequence of frames, here at a frame rate of 4 KHz. Thus, for each frame of data fed to lines 28.sub.0 -28.sub.511 a sequence of digital samples is produced by the I.F.F.T. 26 on line 22 at a sampling rate of about f.sub.s =2.048 MHz (more precisely 2.208 MHz).
The transmitter section 18 of the central office modem 12 also includes: a digital to analog converter (DAC) 30 for converting the sequence of samples of digital samples into a corresponding analog signal on line 32 at the rate f.sub.s =2.048 MHz; and a band pass filter 34, fed by the analog signal and having a pass band extending over the upper portion of the M carrier frequencies, for producing, after passing through a conventional isolation hybrid 36, on the common communication medium 16, the down-stream signal having a band width extending over the upper portion of the M carrier frequencies; here over subchannels 35 through 255 as shown in FIG. 2.
The remote terminal modem 14 includes a receiver section 40 having: a band pass filter 42, coupled to the common communication medium 16 via a hybrid 43, for passing signals in the down-stream signal fed thereto by the central office modem 12 transmitter section 18. As noted above, the data in the down-stream signal extends over the upper portion of the M carrier frequencies (i.e., subchannels 35 through 255, FIG. 2). An analog to digital converter (ADC) 44 is provided for converting the signals passed by the band pass filter 42 into a sequence of digital data on bus 46. The data on bus 46 is produced at the sampling rate, f.sub.s. A demodulator 48 is fed by the sequence of data samples produced by the analog to digital converter 44 on bus 46, for separating such digital data in such samples on bus 46 into the upper portion of the plurality of M carrier frequencies (i.e., into subchannels 1 through 255, FIG. 2, it being understood that only the data in subchannels 35 through 255 are of interest). More particularly, the demodulator 48 includes a Time Domain Equalizer (T.D.Q.)/512 point Fast Fourier Transformer (F.F.T.) 50 and a Q.A.M. decoder 52, arranged in a conventional manner as shown to provide, ideally, a stream of data on line 54 corresponding to the stream of data line 13.
The remote terminal modem 14 includes a transmitter section 60, for distributing a second stream of data fed to the remote terminal modem 14 on line 63 among the lower portion of the plurality, M, 4 KHz wide, carrier frequencies shown in FIG. 2 (i.e., on channels 8 through 31). More particularly, the transmitter section 60 includes a modulator 62, here for receiving the data on line 63 and for distributing such data over subchannels 8 through 31. Here, the modulator 62 includes a Quadrature Amplitude Modulator (Q.A.M.) encoder 64 and an Inverse Fast Fourier Transformer (I.F.F.T.) 66 arranged in as conventional manner as shown. Here, the I.F.F.T. 66 is a 64 point I.F.F.T. Thus, the incoming data on line 63 is selectively encoded by the Q.A.M. encoder 64 at the frame rate, f.sub.r, here 4 KHz and the I.F.F.T. 66 produces for each frame a sequence of digital samples on line 63 at a rate f.sub.s /8=2(M+1)f.sub.r /8. More particularly, the sequence of digital samples on line 63 is encoded by Q.A.M. encoder 64 onto the 64 input lines 68.sub.0 -68.sub.63 of the I.F.F.T. 66 as a sequence of frames, here at a frame rate of 4 KHz. Thus, for each frame of data fed to lines 68.sub.0 -28.sub.63, a sequence of digital samples is produced by the I.F.F.T. 66 on bus 70 at a rate of f.sub.s /8=256 KHz.
The transmitter section 60 of the remote terminal modem 14 also includes: a digital to analog converter (DAC) 72 for converting the sequence of samples of digital samples on bus 70 into a corresponding analog signal on line 74. A lowpass filter 76 is fed by the analog signal and has a bandwidth extending over the lower portion of the M carrier frequencies (i.e., from dc to 128 KHz (i.e., subchannel 31)), for producing, after passing through a conventional isolation hybrid 43, on the common communication medium 16, the up-stream signal having a band width extending over such lower portion of the M carrier frequencies; here over subchannels 8 through 31 as shown in FIG. 2.
The central office modem 12 includes a receiver section 80 having: a lowpass filter 82, coupled, via the isolation hybrid 36, to the common communication medium 16, for passing signals in the up-stream signal fed thereto by the remote terminal modem 14 extending over lower portion of the M carrier frequencies, here over subchannels 8 through 31. An analog to digital converter 84 is provided for converting the signal passed by the lowpass filter 82 into a sequence of digital data on bus 86 at the sampling rate, f.sub.s /8. A demodulator 88 is fed by the sequence of samples produced by the analog to digital converter 84 on bus 86 for combining the digital data in such samples into the lower portion of the M carrier frequencies (i.e., the data in subchannels 8 through 31) into a data stream on line 90 corresponding, ideally, to the data stream on line 63. Here, the demodulator 88 includes a Time Domain Equalizer (T.D.Q.)/64 point Fast Fourier Transformer (F.F.T.) 92 and a Q.A.M. decoder 94. Thus, T.D.Q./F.F.T. 92 separates the digital data fed thereto by the analog to digital converter 84 at rate f.sub.s /8, into frames of data at the 4 KHz frame rate among subchannels 1 through 31 (on lines 95.sub.0 -95.sub.63, respectively); it being understood that the data of interest will appear in subchannels 8 through 31. The data on lines 94.sub.5 -94.sub.31 are combined by the Q.A.M. encoder 94 to produce a properly arranged stream of data on line 90.
It should be noted that because the transform size used for the up-stream signal (i.e., a 64 point I.F.F.T. 66) is different from the transform size used for the down-stream signal (i.e., a 512 point I.F.F.T. 26), the down-stream signal bandwidth of 255 subchannels extends to f.sub.s /2 and the up-stream signal bandwidth of subchannels 8-31 extends to nearly f.sub.s /16. Thus, considering first the up-stream signal produced by the remote terminal modem 14, it is first noted that the up-stream signal has a bandwidth from subchannel 8 through 31, i.e., a frequency spectrum shown in FIG. 3). However, the data produced on bus 70 by modulator 62 also includes images (i.e., aliases) which repeat at the rate that data is produced on bus 70. Here, as noted above, digital samples are produced on bus 70 at f.sub.s /8; a rate greater than twice the highest frequency in the up-stream signal bandwidth. Thus, while the images of the up-stream signal (i.e., subchannels 8 through 31) repeat at the frequency f.sub.s /8, as shown by the shaded spectrum 71 in FIG. 3 and therefore do not alias unto itself, it is noted that such images 71.sub.images extend into the bandwidth of the down-stream signal, i.e., into subchannels 35 through 255, as shown in FIG. 3. The sinx/x, where x is frequency, filter effect of the DAC 72 is shown by the dotted line in FIG. 4A. These un-wanted images 71.sub.images are filtered to some extent by the up-stream transmit low-pass filter 76 and the hold effect of the DAC 72, but they are not removed altogether. The amount of filtering is limited since the use of high-order analog filters for filter 76, in addition to being expensive, also have long impulse responses that introduce excessive intersymbol interference and degrade modem performance. The remaining images pass through the hybrid 43 with some limited attenuation, and appear, as noted above, directly within the band (i.e., channels 35 through 255) occupied by the down-stream signal thereby causing interference. Referring also to FIG. 4B, the frequency spectrum 73 of the down-stream signal is shown as a function of frequency. The images of the up-stream signal are shown shaded in FIG. 4B. It is noted that the attenuation in the down-stream signal increases at higher signal frequencies so that the echo signal from the DAC 72 images can exceed the level of the received down-stream signal. As a result, severe degradation of the down-stream signal to noise ratio (SNR) may result.
A similar problem occurs at the central office modem 18. At the central office modem, the ADC 84 sampling rate, f.sub.s /8, is one-eighth that of the sampling rate, f.sub.s, of the down-stream signal. That is, samples are produced by ADC 84 on bus 86 at a rate, f.sub.s /8, and samples are produced by the I.F.F.T. 26 at the rate f.sub.s. The frequency spectrum of the down-stream signal is shown in FIG. 5A. The frequency spectrum of subchannels 5 through 31 is shown in the shaded region 77 in FIG. 5A. The spectrum of two images of such spectrum at the output of ADC 84 are shown in FIGS. 5B and 5C. Thus, while the up-stream signal is sampled at twice the highest frequency in the bandwidth covering channels 8 through 31 (i.e., the up-stream signal is sampled at twice f.sub.s /16) so that images thereof will not alias onto themselves, it is noted the unwanted portion of the down-stream signal passing into the receiver section of the central office receiver section 80 are sampled at a rate f.sub.s /8; i.e., a rate one-eighth the Nyquist sampling rate required to prevent aliasing of the down-stream signal. Thus, with the up-stream sampling rate, f.sub.s /8, of the ADC 84 the images of the down-stream signal occupy the frequency spectrum of subchannels 8 through 31, i.e., the frequencies of the up-stream signal. Clearly, with limited hybrid 36 attenuation, the down-stream signal will be aliased by the ADC 84 and will fall directly within subchannels 8-31 causing interference with the up-stream signal.